Sample preparation can be minimal or elaborate for SEM analysis, depending on the nature of the samples and the data required. Minimal preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. The choice of material for conductive coatings depends on the data to be acquired: carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of "low vacuum" operation <ref> http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html </ref>.

Sample preparation can be minimal or elaborate for SEM analysis, depending on the nature of the samples and the data required. Minimal preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. The choice of material for conductive coatings depends on the data to be acquired: carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of "low vacuum" operation <ref> http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html </ref>.

Overview

SEM Diagram

Secondary Electron Microscope or SEM was developed in 1938 to image the surface of samples with high resolution. Both a SEM and a light microscope apply the same principle; however, instead of using visible light, the SEM use electrons for imaging. Nonetheless, the wavelength of visible light limits resolution of the images from the optical microscope, while accelerated electrons in the SEM, which has much shorter wavelength, make it possible to investigate features in microscale to nanoscsale with high resolution. Therefore, this instrument has opened doors in numerous fields such as, physics, materials science, biology, chemistry etc.

Scattered electrons

To create an image, a beam of incident electrons(or primary electrons) is generated at the top of the microscope by a thermal emission source, for example, a heated tungsten filament, or a field emission cathode. The energy of the incident electrons can be varied from 100 eV to 30 keV depending on the evaluation objectives. The electron beam follows a vertical path through the vacuum chamber. The beam also passes through electromagnetic lenses which focus the beam down toward the specimen. When the beam hit the specimen, secondary emissions such as electrons, and X- rays are emitted from the specimen to chamber. Secondary electrons, backscattered electrons and X-rays are detected and converted to into a signal to the screens by detectors .

However, the primary limitation of the SEM is that a sample has to be clean, dry and electrically conductive. For non-conductive specimens, they must be coated with a conductive film to prevent charging. Moreover, a high vacuum chamber is required during operating to reach high resolution because the incident beam may interact with atoms in air before hitting the sample [1]..

SEM Sample Preparation

Sample preparation can be minimal or elaborate for SEM analysis, depending on the nature of the samples and the data required. Minimal preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. The choice of material for conductive coatings depends on the data to be acquired: carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of "low vacuum" operation [2].

Note: Introduction to the field of sample preparation for electron microscopy. Basic concepts and tools needed are discussed to be able to perform the simpler types of analysis in electron microscopy.

Available at CAVS

Field Emission Gun (FEG) SEM

The field emission scanning electron microscope(FE-SEM) is an electron microscope which produces the images of the sample by scanning with a high-energy beam of electrons in a raster scan pattern. In conventional SEM, an electron beam is generated by heating a tungsten filament, thus the size of the resulting beam is limited by the geometry of the filament, which limits magnification. In contrast, in the FE-SEM, A cold field emission source generates a electron beam by applying a high voltage to a very sharp point, which is as small as 10 nm, thus the FE-SEM offers high performance and high resolutions. As a result, this FEG-SEM provides higher spatial resolution, better reliability and higher performance than other conventional scanning electron microscopes. However,the FE-SEM requires extremely high vacuum conditions in the chamber [3].

Environmental (EVO) SEM

Scattered electrons

The Environment Scanning Electron Microscope (ESEM) was developed in the mid eighties to overcome the constrains of the conventional SEM that requires high vacuum and dry samples. The ESEM allows researchers to investigate features of a sample in a range of pressure and temperature. The wet, dirty or non-conductive sample can be examined in their natural state without any preparation. The ESEM provides high resolution secondary electron imaging, at pressure as high as 50 Torr, and temperatures as high as 1800 K.

Rather than using a single pressure limiting aperture in conventional SEM, ESEM uses multiple pressure limiting apertures to separate chamber from the column; therefore, pressure as high as 50 Torr can be sustained in the chamber, but the column is still in high vacuum condition. Moreover, the ESEM uses Environmental Secondary Detector(ESD), which can be operated in non-vacuum environment, instead of using Everhart-Thornley(ET) in conventional SEM.

The ESD applies the principle of gas ionization. A positive charge is applied to the detectors to attract secondary electrons emitted by the sample when interacting with a beam of primary electron. The secondary electrons are accelerated in the detector field, thus the they hit gas molecules. The secondary electrons signal are amplified and positive ions are created as a result of gas ionization. For non-conductive samples, the sample surface effectively attracts the positive ions caused by gas ionization as charge accumulates from the beam of primary electron. These positive ions help to prevent charging artifacts [4].

Experimental Capabilities

Electron Backscatter Diffraction (EBSD)

Electron backscatter diffraction (EBSD), known as backscatter Kikuchi diffraction (BKD), is a analytical technique used in Scanning electron microscopic (SEM) to study crystal orientation of bulk materials with high spatial resolution. This technique is applied to obtain highly accurate information about grain orientations, grain boundaries and phase identification. It is also used to study microstructures, texture and defects [5].

To obtain crystalline information, this technique is operated in the SEM that is equipped with an EBSD detector, which usually composes of phosphor screens, compact lens and low light CCD camera. A beam of incident electrons is focused down toward the sample, and channeling of the backscattered electrons creates Kikuchi patterns. After analyzing Kikuchi patterns, the information about particular grain that was hit by the beam of electron can be obtained.

However, the limitations of this technique are that the number of grains that can be investigated in a reasonable time is about 1,000 - 100,000 grains, and there are some difficulties with viewing small grain size in a very thin film (<100nm)[6].

Energy-dispersive X-ray spectroscopy (EDS)

sample of EDS spectrum

Energy Dispersive X-ray Spectroscopy (EDS) is a chemical analysis technique coupled with scanning electron microscopy (SEM) or Transmission Electron Microscopy (TEM). The EDS technique detects X-rays generated from the sample when a beam of electrons hit the surface of the sample in order to collect information about the elemental composition in the particular points. Features as small as 1 µm or less can be analyzed.

When the sample is bombarded by the electron beam, the sample emitted electrons to the chamber, and then electron vacancies are occurred. To balance the energy difference between the two states, an x-ray is emitted. The energy of the x-ray is a characteristic of the element composed on the sample's features.

The EDS x-ray detector detects the relative amount of emitted x-rays versus its energy. The detector is typically a lithium-drifted silicon, solid-state device. When an incident x-ray hits the detector, it generates a charge pulse that is proportional to the energy of the x-ray. The charge pulse is converted to a voltage pulse in order to sent to a computer for displays and analysis[7].

Backscattered Electron (BSE) Imaging

Backscattered electrons (BSE) consist of high-energy electrons originating in the electron beam, that are reflected or back-scattered out of the specimen interaction volume by elastic scattering interactions with specimen atoms. Since heavy elements (high atomic number) backscatter electrons more strongly than light elements (low atomic number), and thus appear brighter in the image, BSE are used to detect contrast between areas with different chemical compositions.[26] The Everhart-Thornley detector, which is normally positioned to one side of the specimen, is inefficient for the detection of backscattered electrons because few such electrons are emitted in the solid angle subtended by the detector, and because the positively biased detection grid has little ability to attract the higher energy BSE. Dedicated backscattered electron detectors are positioned above the sample in a "doughnut" type arrangement, concentric with the electron beam, maximizing the solid angle of collection. BSE detectors are usually either of scintillator or of semiconductor types. When all parts of the detector are used to collect electrons symmetrically about the beam, atomic number contrast is produced. However, strong topographic contrast is produced by collecting back-scattered electrons from one side above the specimen using an asymmetrical, directional BSE detector; the resulting contrast appears as illumination of the topography from that side. Semiconductor detectors can be made in radial segments that can be switched in or out to control the type of contrast produced and its directionality.